#include <bits/stdc++.h>
#include <atcoder/modint>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
//using mint = modint1000000007;
using ll = long long;
using ld = long double;
using pll = pair<ll, ll>;
using tlll = tuple<ll, ll, ll>;
constexpr ll INF = 1LL << 60;
template<class T> bool chmin(T& a, T b) {if (a > b) {a = b; return true;} return false;}
template<class T> bool chmax(T& a, T b) {if (a < b) {a = b; return true;} return false;}
ll safemod(ll A, ll M) {return (A % M + M) % M;}
ll divfloor(ll A, ll B) {if (B < 0) {return divfloor(-A, -B);} return (A - safemod(A, B)) / B;}
ll divceil(ll A, ll B) {if (B < 0) {return divceil(-A, -B);} return divfloor(A + B - 1, B);}
#define FINALANS(A) do {cout << (A) << '\n'; exit(0);} while (false)

int main()
{
  ll L, R;
  cin >> L >> R;

  ll N = 2 * R + 10;
  vector<bool> isprime(N, true);
  isprime.at(0) = false, isprime.at(1) = false;
  for (ll p = 2; p * p < N; p++)
  {
    if (isprime.at(p))
    {
      for (ll k = 2; k * p < N; k++)
      {
        isprime.at(k * p) = false;
      }
    }
  }

  ll ans = 0;
  for (ll A = L; A <= R; A++)
  {
    if (isprime.at(A))
      ans++;
    if (A != R && isprime.at(2 * A + 1))
      ans++;
  }
  cout << ans << endl;
}